The river is definitely a good possibility, but those Midwestern farms are so vast (and much of the private land in the area was never searched), his remains probably wouldn't have been found.
I think a lot of missing people who vanished in rural or deeply forested areas in America probably got lost, died of natural causes, and their bodies are still undiscovered. Kyron Horman's school was located in the middle of a big, really dense forest which wasn't searched until that evening.
There are so many cases of bodies lying undiscovered for months or even years, even after being searched, and the US is just so massive and has such dense areas of wilderness, even farmlands (which of course isn't wilderness) are just so huge.
I found out something creepy the other day - in certain states in the US there's a law known as the purple paint law, where if you daub your fenceposts with a splotch of paint in a particular shade of purple, that means you do not tolerate trespassers, it's a legal declaration of your right to privacy and your right to enforce your right to privacy at all costs, and it's more or less a declaration that if you continue, you'll get shot and it'll be on you because you ignored the warning.
I think there's a huge US-UK cultural divide, in that we live on this tiny very crowded island with lots of green spaces but not vast amount of dense forest or wilderness (Scotland has the most wilderness but also has Right to Roam) so the way we conceptualise privacy is very different from the US which has so much space. And we have so many public footpaths, Right to Roam, etc. Go to any MN thread about farming or just rural life and posters will be complaining about tourists or random people walking on their land or entering fields, trying to stroke horses, etc. In Britain and lots of mainland Europe it's just the culture to sort of regard all rural land as being almost semi-public, regardless of the law. Whereas in rural parts of the US, you just grow up knowing never to step foot on someone else's land or unknown land without permission because a lot of people are insanely protective of their privacy and regard anyone stepping foot onto their property (even just to try to ring their doorbell) as a hostile act.
Large swathes of forested land where Maura Murray went missing have never been searched because the landowners wouldn't give permission, not necessarily because they have something to hide (they can't all have something to hide), but because rural New Hampshire where she vanished is just really infamous for being full of people who are intensely protective of their privacy and very strongly against any form of government interference, and wouldn't ever let police onto their property without a warrant as a point of principle.